Photovoltaic devices, i.e., solar cells, are capable of converting solar radiation into usable electrical energy. The energy conversion occurs as the result of what is known as the photovoltaic effect. Solar radiation impinging on a solar cell and absorbed by an active region of semiconductor material generates electrons and holes. Electrons and holes are separated by a built-in electric field in the solar cell, such as a rectifying junction. The separation of electrons and holes by the rectifying junction results in the generation of an electric current known as the photocurrent and a voltage known as the photovoltage. The electrons flow toward the region of semiconductor material having N-type conductivity, and the holes flow toward a region of semiconductor material of opposite conductivity or a metal layer.
Significant reduction in the cost of solar cells can be achieved by reducing the materials and manufacturing cost and by improving the photovoltaic conversion efficiency. Some progress has been made in the identification of several promising thin film solar cells such as CdTe, Zn.sub.3 P.sub.2, Cu.sub.2 Se and amorphous silicon. Unfortunately, the cell efficiencies achieved in thin film devices are significantly lower than those obtained for single crystal cells usually because of the lower quality film epitaxy produced on low cost foreign substrates. Thus, heretofore a trade-off has existed in the selection of solar cell substrates with either amorphous or small grain materials such as glass, ceramic or metal sheets which have a low cost but hinder the growth of high quality solar cells or large grain or single crystal material which allow the growth of high quality solar cell layers but are higher in cost.
The subject invention avoids the above dilemma by enabling the use of low cost substrate material such as synthetic organic resins, graphite, glass, abundant metals or other crystalline materials having grain sizes not greater than about 1 micron by depositing a low melting point crystalline film on the substrate and recrystallizing the film to form a large grain material on which can be grown the solar cell or semiconductor material.